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Update: I wanted to provide a brief summary of what you will find below. In short, the government wants an equivilent MPG standard for electric cars that goes back to the power plant to estimate that amount of fossil fuels must be burned to create the electricity that fills the batteries of an electric car. The EPA's methodology is flawed because it assumes perfect conversion of the potential energy in fossil fuels to electricity, an assumption that violates the second law of thermodynamics. The Department of Energy has a better methodology that computes electric vehicle equivalent mileage based on real world power plant efficiencies and fuel mixes, while also taking into account energy used for refining gasoline for traditional cars. Using this better DOE methodology, we get MPGe's for electric cars that are barely 1/3 of the EPA figures.

In a parking lot near Detroit, General Motors is accumulating Chevy Volts. They cannot yet be sold to the public because the Environmental Protection Agency (EPA) has not yet delivered the required mileage sticker for the window. We can, though, get a preview of how the EPA may complete this admittedly tricky task from the mileage numbers they recently assigned to the all-electric Nissan Leaf. The Leaf's numbers make it clear that this Administration is willing to resort to accounting fraud to promote its automotive agenda.

The traditional metric for automobile efficiency is miles per gallon (MPG) of gasoline used. But what does MPG even mean for a vehicle that doesn't burn liquid fuel? Most of us would agree that the Leaf certainly does use fuel -- after all, something must create the electricity, and in the US that is generally some sort of fossil fuel. So theoretically we should be able to create an MPG equivalent, or MPGe for short, to measure the fossil fuel use of electric vehicles.

For more than a decade, within the EPA and the Department of Energy (DOE), a number of different approaches to this problem have been discussed. With the release of the numbers for the Nissan Leaf, we now know what approach the EPA is taking, and results are depressing for those of us who would like to see transparency and adherence to science in the Administration. This Administration has a huge intellectual investment in electric vehicles and financial investment in the Chevy Volt. Like a shady company trying to pump up their stock by choosing a series of highly questionable accounting conventions, the EPA has chosen an approach that grossly overestimates the MPGe of electric vehicles.

The Nissan Leaf was rated at 99 MPGe. To reach this number, the EPA created a conversion factor between a quantity of electric energy, measured in kilowatt-hours (KwH) and a volume of gasoline, measured in gallons. They did this be dividing the potential energy or heating value of a gallon of gasoline (115,000 BTUs) by the energy in a KwH of electricity (3412 BTUs) to get a conversion factor of 33.7 gallons per KwH. Using this factor, they can convert miles per KwH of electricity in an electric vehicle to an MPGe that is supposedly comparable to more traditional vehicles.

The problem is that, using this methodology, the EPA is comparing apples to oranges. The single biggest energy loss in fossil fuel combustion is the step when we try to capture useful mechanical work (ie spinning a driveshaft in a car or a generator in a power plant) from the heat of the fuel's combustion. Even the most efficient processes tend to capture only half of the potential energy of the fuel. There can be other losses in the conversion and distribution chain, but this is by far the largest.

The EPA is therefore giving the electric vehicle a huge break. When we measure mpg on a traditional car, the efficiency takes a big hit due to the conversion efficiencies and heat losses in combustion. The same thing happens when we generate electricity, but the electric car in this measurement is not being saddled with these losses, even though we know they still occur in the system.

Lets consider an analogy. We want to measure how efficiently two different workers can install a refrigerator in a customer's apartment. In both cases the customer lives in a fourth floor walkup. The first installer finds the refrigerator has been left on the street. He has to spend much of his time struggling to haul the appliance up four flights of stairs. After that, relatively speaking, the installation is a breeze. The second installer finds his refrigerator has thoughtfully been delivered right to the customer's door on the fourth floor. He quickly brings the unit inside and completes the installation.

So who is a better installer? If one only looks at the installer's time, the second person looks orders of magnitude better. But we know that he is only faster because he offloaded much of the work on the delivery guys. If we were to look at the total time of the delivery person plus the installer, we'd probably find they were much closer in their productivity. The same is true of the mileage standards -- by the EPA's metric, the electric vehicle looks much better than the traditional vehicle, but that is only because someone else at the power plant had to do the really hard bit of work that the traditional auto must do itself. Having electricity rather than gasoline in the tank is the equivalent of starting with the refrigerator at the top rather than the bottom of the stairs.

An apples to apples comparison, then, would compare the traditional car’s MPG with the Leaf’s miles per gallon of gasoline (or gasoline equivalent) that would have to be burned to generate the electricity it uses. Incredibly, the DOE actually established and published such a standard in a rules-making process way back in Clinton Administration. The standard, called "well to wheels," adds a couple new factors to the MPGe calculation we discussed above.

First, the DOE looked at the electrical generation efficiency, and determined that only 32.8% of the potential energy in the fossil fuel becomes electric energy in the average US power plant, which it further reduced to 30.3% to account for transmission losses. However, they realized it was unfair to charge electric vehicles for these losses without also charging gasoline-powered vehicles for the energy cost of refining and gasoline distribution. They calculated these as adding 20% to the energy it takes to run a gas-powered car, but rather than reducing existing MPG standards by this amount, they instead gave a credit back to electric vehicles. The 30.3% electric production and distribution factor was increased to a final adjustment factor of 36.5%. This means that the conversion factor discussed above of 33.7 gallons/KwH must be multiplied by 36.5% to get a true apples to apples MPGe figure.

The end result is startling. Using the DOE's apples to apples methodology, the MPGe of the Nissan Leaf is not 99 but 36! Now, 36 is a good mileage number, but it is pretty pedestrian compared to the overblown expectations for electric vehicles, and is actually lower than the EPA calculated mileage of a number of hybrids and even a few traditional gasoline-powered vehicles like the Honda CR-Z.